The present disclosure relates to a fuel cell using an oxidoreductase, and an enzyme electrode for use therein.
A fuel cell fixedly provided with an oxidoreductase on an electrode for use as a catalyst (hereinafter, referred to also as enzyme cell) is receiving attention as a next-generation fuel cell with a large capacity and an excellent level of safety. This is because, with such a fuel cell, electrons can be extracted with a good efficiency from a fuel of glucose, ethanol or others that are not available for use as general industrial catalysts.
With an enzyme cell using glucose as a fuel, like a reaction scheme shown in FIG. 27, the oxidation reaction of the glucose (Glucose) takes place at the anode, and at the cathode, the reduction reaction of oxygen (O2) in the air takes place. At the anode, electrons are directed to pass through in order of glucose, glucose dehydrogenase, nicotinamide adeninedinucleotide (NAD+), diaphorase, an electron mediator, and an electrode (carbon). On the other hand, at the cathode, the electrons emitted from the anode are directed to pass through in order of an electrode (carbon), an electron transfer mediator, and bilirubin oxidase (BOD). The reduction reaction is then taken place by these electrons and the oxygen coming from the outside so that electrical energy is generated.
A biological fuel cell exemplified by such an enzyme cell has several problems for practical use thereof. For example, as to any previous biological fuel cell, the power to be produced thereby is smaller than that by any other types of fuel cells. Therefore, in order to obtain the higher power, increasing the capacity of the cell is a requirement as well as configuring the cell like a layer-built cell. In addition, the fuel of the biological fuel cell is generally in the liquid state and is very viscous. There is a possibility of leakage of fuel because the fuel is in the liquid state, although is very viscous. If the container for storage of the fuel is sealed tighter for the purpose of preventing such a fuel leakage, this causes a problem of difficulty in supplying the fuel to the inside of the cell because the fuel is high in viscosity.
In consideration thereof, in recent years, in order to solve such problems related to the biological fuel cell, various studies have been conducted (refer to Patent Literatures 1 and 2). Patent Literature 1 describes a button-shaped biological fuel cell, which includes a cathode, a proton conductor, and an anode that are disposed one on the other in this order. The resulting layer-built structure is sandwiched between a cathode current collector and an anode current collector. The cathode current collector is formed with a supply port of an oxidizing agent, and the anode current collector is formed with a supply port of a fuel. In such a fuel cell, the outer edge of the cathode current collector is caulked to the outer periphery portion of the anode current collector via a gasket to make uniform the pressure to be imposed on the components, and to increase the degree of contact among the components, thereby preventing variations in the power and leakage of the fuel.
Patent Literature 2 describes an enzyme cell, which is aiming to increase the output current or the output voltage by providing a plurality of cell portions in a cell. FIG. 28 shows the configuration of such a previous enzyme cell described in Patent literature 2. This enzyme cell 100 is provided with cell portions 115 and 116. The cell portion 115 is configured by a cathode 103, a proton conductor 104, and an anode 105. The cell portion 116 is configured by an anode 109, a proton conductor 110, and a cathode 111. These cell portions 115 and 116 are disposed with a spacer 107 sandwiched therebetween. In such a manner as to enclose the anodes 105 and 109, the anode current collectors 106 and 108, and the spacer 107, a fuel storage container 114 is provided. To the outside of the cathodes 103 and 111, the cathode current collectors 102 and 112 are respectively disposed, and to the outside thereof, spacers 101 and 103 that can pass therethrough the air are provided.
With such an enzyme cell 100, the anodes 105 and 109 are each fixedly provided with an enzyme, and when the fuel storage container 114 is filled with a glucose solution for use as a fuel, at the anodes 105 and 109, electrons are extracted by the glucose being decomposed by the enzyme, and protons (H+) are generated. At the cathodes 103 and 111, water is generated by the reaction of H+ and the electrons with the oxygen in the air. The H+ are those transported through the proton conductors 110 and 104, and the electrons are those provided via an external circuit after being extracted at the anodes 105 and 109. When a load is connected between the cathode current collectors 102 and 112 and the anode current collector 106, the load is provided therethrough a flow of current being the sum of the output currents of the two cell portions 115 and 116. Thus, the resulting output current and voltage can be larger than those in the previous enzyme cell.